1. Field of the Invention
This invention relates to a new and improved fluid sample cell for spectroscopic analysis of a wide variety of different fluid samples.
2. Description of the Prior Art
A wide variety of sample cells are known for use in accordance with conventional transmission spectroscopy techniques for analyzing fluid samples to determine the concentration of radiation energy-absorbing sample constituents. Such analysis is based on the selective attenuation of radiation energy at different predetermined wavelengths passing through the sample in accordance with the absorption characteristics of the sample, the concentration of the radiation energy-absorbing constituents of the sample, and the radiation energy transmission path length through the sample. Certain fluid samples, for example, liquids, contain suspensions of fine particles which independently attenuate radiation energy transmitted therethrough by diffusely scattering the same. Depending upon particle size distribution and concentration of particles in the fluid medium, a portion of the incident beam of light, instead of being directly transmitted therethrough, will be diffusely scattered into forward- and back-scatter components of variable geometric distribution. In conventional transmission spectroscopy, the back-scatter component, as well as a portion of the forward-scatter component, will not be sensed by the radiation sensor. The resultant loss of signal appears as part of the attenuation measurement. Variations in turbidity, therefore, will introduce a variable parameter in the measurement due to variations in the loss of the diffusely scattered components, as opposed to variations in the purely absorptive properties of the constituent of interest in the fluid.
Furthermore, the spectral energy attenuation characteristics of true solutions is a logarithmic function of molecular concentration, whereas the diffuse scattering of light by finely suspended particulate matter is a non-linear function of particle size and concentration as well as wavelength. Such suspension, therefore, will introduce an error in the analysis results, unless time-consuming and complex (and oftentimes inaccurate) corrections are made for the diffusely scattered radiation energy components. In addition, a wide variety of fluid samples, for example, heavy process food syrups, ice creams or the like, do not transmit sufficient radiation energy to be appropriately processed in accordance with conventional transmission spectroscopy techniques and, hence, require special sample processing, such as dilution, with attendant introduction of further possibility of error.
Also, a wide variety of sample cells are known for use in the analysis of generally solid samples by conventional reflectance spectroscopy techniques. By such techniques, solid samples, generally reduced to a powdered or finely ground consistency, are illuminated by a source of spectral radiation and analyzed for their surface spectral reflectance properties. Light striking and penetrating the sample surface is partially absorbed in accordance with the concentration of sample constituents and their spectral absorbance characteristics; also, such light is diffusely scattered in a similar manner as with turbid liquid samples, into forward and back-scatter components. It is the back-scatter component, selectively attenuated by sample spectral absorbance characteristics, which is measured in a reflectance instrument and used to determine constituent concentration. However, the forward-scatter component is lost by absorbtion within the sample and, hence, not available to determine constituent concentration, although such component contains worthwhile information. Such sample cells would be generally inapplicable for use in the analysis of fluid samples, wherein the spectral reflective surface properties per se of the sample are not indicative of, for example, the concentration of a particular sample constituent.
In addition, and because of the basic differences in optical requirements between transmission and reflectance spectroscopy, currently available spectroscopy instruments are, in general, limited to only one type of measurement or would require complicated and costly accessory equipment for conversion from one type of measurement to the other. Also, because of the limitations set forth hereinabove, a wide variety of existing semi-liquids or semi-solids simply cannot be conveniently analyzed with suitable precision by either existing transmission spectroscopy or reflectance spectroscopy instruments.